The invention relates to a device for delivery of a pharmaceutical to the systemic circulation of a human or animal body via the skin, and in particular, via skin from which the epidermis has been removed.
The administration of pharmaceuticals through the skin is a concept which is now well-established. Transdermal administration has several advantages over more conventional forms of drug delivery such as injection or oral ingestion. Transdermal devices can provide sustained and controlled release of the active agent over a prolonged period so that the resulting blood levels remain constant. This is in contrast to administration, by injection, for example where surges of the agent occur in the bloodstream immediately after administration and then drop away rapidly until the next dose is given. In the case of oral administration the blood level is further influenced by the contents of the intestines and is therefore difficult to control. Transdermal administration permits direct access to the bloodstream without first passage through the gastro-intestinal tract and liver. In addition, transdermal administration is more convenient and comfortable for patients because a small device or plaster can remain attached to the skin for a prolonged period without patient intervention. The inconvenience of administering drugs at regular intervals orally or by injection is therefore avoided.
Despite the aforementioned advantages, the number and type of pharmaceutically active agents which have been successfully given transdermally is limited. This is because the epidermis provides a natural barrier against the ingress of foreign substances into the body and there are few drugs which are able to adequately permeate this barrier of their own accord. One approach to this problem has been to include in transdermal devices so-called permeation enhancers, molecules which are able to increase the uptake of an active agent through the skin. A particular example is diethyltoluamide although many others are known to the skilled man in this field. The use of permeation enhancers has increased the number of drugs which may be given transdermally and because of the lipophilic nature of the epidermis there has been reasonable success with drugs which are relatively hydrophobic molecules. The steroid hormones are a particular example. In addition to the use of permeation enhancers, electrochemical means have also been used to promote drug transport across the epidermis with some success.
However, notwithstanding these methods the epidermis is a very effective barrier against hydrophilic substances which has hitherto rendered transdermal administration of drugs falling in this category very difficult, if not impossible. Thus the increasing number of potent pharmaceuticals which are proteins and polypeptides cannot generally be administered transdermally.